1. Every time a player logs in, remove that player from all four seasonal Game Stages (SpringStage, SummerStage, FallStage, WinterStage)

2. Check the season via the Serene Seasons command

3. Based on the result from #2, add the player that just logged in to one of those four stages?

I'm trying to figure out a way I can keep all my players synced to seasonal Game Stages. If I don't sync it at login, then any player who happens to not be online when the sync happens won't be properly synced, so they'll be seeing the wrong mobs, etc, because their Game Stages won't match the actual season.

Actually I went to read up a bit more on the wiki for the Mod, there is a "player_log_in" event trigger, so you should be able to. Still creative use of stages all the same, I'll have to remember that.

React is not particularly good at interaction with other mods' features, but this solution might nonetheless work. Sorry about the (very) late reply, I haven't been available recently. Hopefully you haven't given the idea up yet.

1. This part is easy enough. Listen to the player_log_in event, and use the server_command action to execute whatever command Game Stages has to remove a player from a stage.

player_log_in=[
{
action="server_command" # Executes a command on the server.
parameters {
command="gamestages remove_player_from_stage winter %%%player.uuid%%%" # Replace with whatever the actual command to remove a player from a stage is. Use %%%player.uuid%%% where you'd otherwise use the player's UUID in the command.
}
}
# ... the same for the other stages
]

2 & 3. If the Serene Seasons command returns the season name in a chat message, you can look for the message using the server_chat event. The event is accompanied by a message condition, which can be used to check the contents of the chat message. If the message is "The current season is winter." (or something similar), add the player to the winter stage. Do the same for the other stages. There is no way to verify that the message was actually sent by Serene Seasons, but players will not be able to imitate it, as their messages are prefixed with their names, so this shouldn't be a problem unless other mods send the same message.

server_chat=[ # Fires on the server when a chat message is sent.
{
action="server_command" # Executes a command on the server.
target="every_player" # There is no way to know which player triggered the season message, so the command must be run for every player.
conditions {
message="The current season is winter." # Replace with whatever the output of the Serene Seasons command is.
}
parameters {
command="gamestages add_player_to_stage winter %%%player.uuid%%%" # Replace with whatever the actual command to add a player to a stage is. Use %%%player.uuid%%% where you'd otherwise use the player's UUID in the command.
}
}
# ... the same for the other stages
]

This can be done using the server_command action with the give command. For example, this would give any player who says diamond in the chat a diamond:

server_chat=[ # Fires serverside whenever a chat message is received. { action=server_command # Executes a command as the console. parameters { command="give %%%player.uuid%%% minecraft:diamond" # The give command gives the player an item. The %%%player.uuid%%% part is a property, and essentially inserts the UUID of the event's player into the parameter before using it. } conditions { message=".*diamond.*" } }]

This looks like a really nifty mod! After looking at the documentation, I'm still a bit confused, though. Would it be possible to use this in conjunction with something like HQM in order to trigger a quest completion after placing a certain number of a specific block?

Could I use this mod to detect when a player inputs a "custom command" and it it run another command (that they normally wouldn't be able to run)

As an unrealistic example, if someone type "/president somePlayerName" it would run the command "/op somePlayerName" but the person couldnt just simply run "/op somePlayerName" due to not having the perms to do so?

Would it be possible to make mobs only take damage from certain things using this? Let's say I wanted to make zombies immune to everything besides gold swords, or spiders immune to everything besides a potion of harming.

The former is doable. The latter is trickier, as Minecraft doesn't differ between damage from potions of harming and damage from certain mobs (guardians and evokers). A workaround is to check if the attacker is a player, although that means potions of damage thrown by other mobs/dispensers won't work.